The following points highlight the eighteen steps involved in the protein synthesis in plants.
(i) DNA in the nucleus or nucleoid in prokaryotes directs the synthesis of m-RNA and provides it with necessary genetic information in the form of codons for the formation of specific proteins. This process is called as transcription. m-RNA is synthesized in the presence of the enzyme KNA-polymerase.
(ii) m-RNA molecule moves into the cytoplasm where it causes formation of specific t- RNA molecules having specific anticodons complementary to its codons.
(iii) m-RNA becomes associated with the ribosome which acts as template for protein synthesis. At this template energy is supplied by GTP (Guanosine triphosphate). Mg++ ions are also required.
(iv) t-RNA molecule picks up a specific amino acid by its C-C-A end (according to its anticodon) after the amino acid has been activated by ATP in the presence of a specific enzyme (Fig. 9.21).
Initiation of polypeptide chain:
(v) In bacteria e.g. Escherichia coli the 70 S ribosome dissociates into 30 S and 50 S subunits when Mg++ conc. is low. In higher plants the 80 S ribosome breaks into 40 S and 60 S subunits.
Initiation factor called IF3 is also required for dissociation of 70 S ribosome into 30 S and 50 S subunits.
(vi) 30 S subunits of the ribosome recognises the 5′ terminal end of the m-RNA from where the protein synthesis i.e., the formation of polypeptide chain starts.
Factor IF3 is also required for binding of 30 S subunit with 5’end of m-RNA.
(vii) 30 S subunit also recognises t-RNA—amino acid complex which is then transferred to 50 S subunit. The first amino acid to be incorporated into the polypeptide chain is N- Formlymethionine for which the codons are UUG, AUG, GUG. Thus t-RNA-N-Formly-methionine-complex is the initiator of the polypeptide chain.
Factor IF2 is required for binding of formyl-methionine-t-RNA with 30 S-mRNA complexes.
(viii) 30 S and 50 S subunits of the ribosome unite in the presence of Mg++ ions and become associated with m-RNA. GTP provides energy.
Association of 30 S and 50 S subunits also requires initiation factor IF2.
(One more initiation factor IF-1 is involved in initiation of polypeptide chain but its exact role is not clearly understood).
(ix) t-RNA-amino acid complex attached to the ribosome is placed opposite the specific codon on m-RNA molecule due to the presence of its complementary anticodon in t-RNA molecule.
Elongation of polypeptide chain:
(x) Ribosome and the m-RNA move relative to each other. When ribosome reaches a second codon, another specific t-RNA-amino acid-complex is attached to the ribosome so that its anticodon is placed opposite its complementary codon (Fig. 9.22).
Each ribosome has two sites for the attachment of aminoacyl-tRNA complex. These are ‘P’ site (Peptidyl site, facing towards ‘5’ end of m-RNA) and ‘A’ site (aminoacyl-site, facing towards, 3′ end of m-RNA). It is not clear whether the first aminoacyl-tRNA (i.e., formyl-methionine-tRNA) comes to A site or P site, but ultimately it has to be shifted to P site before a second aminoacyl-tRNA attaches itself to A site.
T-factor (T) or transfer factor (TFI) also called as elongation factor (EF) which consists of two components EF-Tu (unstable on heating) and EF-Ts (stable on heating) is required for proper binding of aminoacyl-tRNA to ‘A’ site of the ribosome.
(xi) A peptide bond is established between the carboxylic and amino group of the two amino acids in the presence of the enzyme peptide synthetase. The preceding t-RNA-amino acid complex breaks so that t-RNA molecule is released into the cytoplasm to become charged again with amino acid.
The ‘P’ site of ribosome now becomes free and ‘A’ site of ribosome carries the growing polypeptide chain.
The peptidyl-tRNA (i.e., the t-RNA carrying the growing polypeptide chain at site ‘A’ is now trans located or shifted to ‘P’ site of ribosome so that its ‘A’ site becomes free paving way for next aminoacyl-tRNA complex to be attached at ‘A’ site.
This transfer of peptidyl-tRNA from A to P site requires GTP and elongation factor called EF- G (earlier known as transfer factor II i.e., TF-II or translocate).
(xii) In the same way, during the movement of the m-RNA and ribosome, a number of specific amino acids are added one after another into the growing polypeptide chain.
(xiii) Polypeptide chain remains attached to the 50 S subunit till it is complete.
Termination of polypeptide chain:
(xiv) Termination of the polypeptide chain takes place when ribosome (to which the polypeptide chain is attached) comes over the non-sense codon in m-RNA molecule during its movement against m-RNA. Non-sense or chain termination codons are UAA, UAG, and UGA.
The termination codons are recognised by two releasing factors RF-1 and RF-2 which in turn are stimulated by another factor called RF3.
(xv) The movement of the ribosome away from the 5′ terminal end of the m-RNA allows the progressive attachment of the other ribosomes so that a number of them may be associated with the same m-RNA molecule and bearing incomplete polypeptide chains of different lengths. This grouping of a number of ribosomes together has been termed as polysome.
(xvi) Polypeptide chains produced on the same m-RNA may be of one or more types depending upon whether m-RNA is monocistronic or polycistronic (i.e., having genetic information for the formation of one or many types of protein molecules).
(xvii) After formation of polypeptide chains the m-RNA disintegrates and ribosome again dissociates into its subunits which are cycled back into the cytoplasm.
(xviii) The process in which a particular nucleotide sequence on m-RNA is translated into a particular amino acid sequence with the help of the ribosomes is called as translation.
As is obvious, the preceding account of protein synthesis in plants pertains mainly to those of bacteria (prokaryotes). It is similar in eukaryotes also but with many differences chief of which are: 1. In eukaryotes 80 S ribosome dissociates into 40 S and 60 S subunits, 2. Instead of formyl-methionine, the first amino acid to be incorporated into the ploypeptide chain is methionine. 3. At least 9 initiation factors are involved in initiation of polypeptide chain as against only 3 in prokaryotes. These nine factors are designated as eIFl-eIF9 (e for eukaryotic).